I bought a '90 Runner, 120K miles, the headgaskets had been done by the dealer. At ~140K miles, burned #6 exhaust valve. Pulled the heads, a well respected independent import machine shop rebuilt them and talked me into using an after market gasket set. Blew at #6 within 10K miles. The gasket maker claimed a manufacturing issue, offered new gaskets or refund, took the refund and installed dealer gaskets.
It continued to reliably make all of the gas mileage of a V8 with none of the power, typical 3.slo. I sold after getting the LX450 with about 220K miles. It was a pretty good truck, but the LX450, way out wheels it, gets the same or better gas mileage, is bigger/more room/more comfortable and is way faster.
The "problem" as I have heard it: When this motor was designed, asbestos was the gasket material of choice. It was one of the best materials ever, took a huge beating, allowed for relatively thin gasket margins/surface area, etc. It was outlawed for gasket use in mid '89, leaving manufactures in a lurch. They obviously could not redesign all of their motors, so had to scramble to find a new material, this was more successful on some motors than others, with the 3.slo being one of the worst. This problem affected lots of motors through the early/mid nineties, including the 80 series.
I like the 2nd gen Runners, with a straight axle conversion they make great wheelers. A couple that I wheel with have 3.4L swaps, makes for a great rig, better power, mileage and very reliable. Two more local rigs are now getting the conversion, both bought wrecks and are swapping motors.